Ketosis in patients with type 2 diabetes mellitus (T2DM) is overlooked due to atypical symptoms. The objective of this study is to evaluate the value of hemoglobin A1c (HbA1c) as a screening tool for ketosis in T2DM patients. This retrospective study consisted of 253 T2DM patients with ketosis at Shanghai 10th People's Hospital during a period from January 1, 2011 to June 30, 2015. A control group consisted of 221 T2DM patients without ketosis randomly selected from inpatients during the same period. Receiver operating characteristic curve (ROC) analysis was used to examine the sensitivity and specificity of HbA1c as an indicator for ketosis. Higher HbA1c levels were correlated with ketosis. In patients with newly diagnosed T2DM, the area under the curve (AUC) was 0.832, with 95% confidence interval (CI) 0.754-0.911. The optimal threshold was 10.1% (87 mmol/mol). In patients with previously diagnosed T2DM, the AUC was 0.811 (95% CI: 0.767-0.856), with an optimal threshold of 8.6% (70 mmol/mol). HbA1c is a potential screening tool for ketosis in patients with T2DM. Ketosis is much more likely with HbA1c values at ≥10.1% in patients with newly diagnosed T2DM and HbA1c values at ≥8.6% in patients with previously diagnosed T2DM.